1. Field of the Invention
This invention relates to a pattern projecting method adapted for projecting a fine circuit pattern of a semiconductor device onto a wafer, and particularly to the method for avoiding deterioration of depth of focus dependent upon the pattern and irregularity in illuminance.
2. Description of the Related Art
In manufacture of the semiconductor device, the 16 MDRAM based upon the design rule of half micron (0.5 .mu.m) level has already been on a mass production line. In research on the semiconductor device, processing of sub-half micron (up to 0.35 .mu.m) level required for the 64 MDRAM and quarter micron (0.25 .mu.m) level required for the 256 MDRAM has been studied. An essential technique for such fine processing is photolithography. The resolution of an exposure device, particularly of a reduced projection exposure device (referred to as a stepper) used for photolithography, determines the possibility of research, development and mass production of the semiconductor device.
Conventionally, the resolution in photolithography has been improved by shortening the exposure wavelength and increasing the numerical aperture of a projection lens of the stepper. This improvement is apparent from the relation shown by the following Equation I, known as Rayleigh's formula: EQU R=k.sub.1 .times..lambda./NA Equation I
wherein R denotes the resolution, k.sub.1 the process coefficient, .lambda. the exposure wavelength, and NA the numerical aperture of the projection lens of the stepper.
In manufacturing the semiconductor device, the resolution (R) and the depth of focus (DOF) are also important parameters for the following reason. Since the wafer surface onto which a pattern is to be projected has irregularities generated therein by warping of the pattern or the wafer, the photoresist coating film cannot be exposed on the same single focal plane at every position in the wafer surface or in the same chip. The depth of focus (DOF) is expressed by the following Equation II: EQU DOF=k.sub.2 .times..lambda./(NA).sup.2 Equation II
wherein k.sub.2 denotes the process coefficient.
The depth of focus (DOF) of approximately 1.5 .mu.m is preferred in the mass production.
From the above Equation II, it is found that the resolution (R) is limited up to about 0.35 .mu.m to achieve the DOF of 1.5 .mu.m using a KrF excimer laser beam with a wavelength .lambda.=248 nm. Stated differently, the required resolution and depth of focus are in the relation of trade-off. It is therefore extremely difficult to resolve a line width finer than 0.35 .mu.m while achieving the focal depth of not less than 1.5 .mu.m.
Thus, it has been attempted recently to eliminate the above difficulty by improving the optical system of the stepper. For improving the optical system, a known technique is used in which darkening the center of an effective light source plane or a pupil plane conjugate thereto of the projection lens causes a super-resolution phenomenon, which improves the contrast of an image. This technique is effective in a partly coherent image-forming system such as the stepper.
One technique to generate the super-resolution phenomenon is an oblique illumination method, or a modified illumination method. In this method, a filter is mounted on a fly eye lens located between an exposure light source and a mask in the stepper optical system, so that the mask is obliquely illuminated.
In the oblique illumination method, the exposure light is incident on the mask obliquely and away from the optical axis. The incident light is divided into a 0-dimensional diffracted light which has not been diffracted by a Cr pattern on the mask and has advanced straight, a .+-.1-dimensional diffracted light diffracted by the Cr pattern, and a .+-.n-dimensional diffracted light of higher dimension. When the exposure light is radiated onto the mask from such an angle that the 0-dimensional diffracted light passes the outer periphery of the incidence pupil, light components on one side of a diffracted light symmetrically generated around the 0-dimensional diffracted light are incident on the reduced projection lens. With the conventional method, since the 0-dimensional diffracted light is incident vertically in the direction of optical axis, only the diffracted light with a diffraction angle of up to .alpha. can be incident on the reduced projection lens. On the contrary, with the oblique illumination method, the diffracted light with a diffraction angle of up to about 2.alpha., though on one side of the 0-dimensional diffracted light, can be incident on the reduced projection lens. Thus, the pitch of interference fringe is diminished, so that the critical resolution can be improved.
Meanwhile, in the conventional exposure method for vertically illuminating the mask, deviation of the wafer along the optical axis from the focal plane, that is, de-focusing, causes an optical path difference in the .+-.1-dimensional light, thereby limiting the depth of focus. In the oblique illumination method, however, if the angle of oblique illumination, that is, the angle of incidence of the 0-dimensional diffracted light, is optimized in response to the pitch of the Cr pattern, angles made by the center line of the reduced projection lens with the 0-dimensional light and the +1-dimensional light, respectively, can be made equal. Thus, the optical path difference can be eliminated. Since the 0-dimensional light and the +1-dimensional light can interfere constantly with the same phase on the wafer, the depth of focus increases, and a satisfactory image-forming status can be maintained even in the case of de-focusing.
However, an illuminating light incident in the direction along the longitudinal direction of a cyclic pattern does not improve the resolution to the cyclic pattern. Thus, to reduce the dependency upon the pattern direction, the effective light source is formed in a circular zone shape, or divided into two parts (left and right parts) or into four parts (upper left and right, and lower left and right parts).
The division into four parts improves the resolution by cutting the vertical incident light on lines and spaces in a particular direction. For instance, a method of controlling the shape of the effective light source using a four-hole filter 10 having a total of four circular apertures 2 located in the first to fourth quadrants of a disc 1, respectively, as shown in FIG. 1, is disclosed in the JP Patent Laid-Open (KOKAI) Publication No.4-267515.
As described above, the oblique illumination method is notable for being capable of improving the critical resolution and the depth of focus in comparison to the conventional method, though using the conventional reticle. However, it also has problems to be solved for practical use.
Since the filter mounted on the fly eye lens filters a light, the illuminance on the wafer surface is lowered. This lowering of illuminance may cause generation of a dimensional change difference within the wafer surface or a severe reduction in throughput when a chemical amplification resist material is used in excimer laser lithography, in which a highly sensitive photoresist has not yet been developed.
The fly eye lens is originally an optical part for transmitting a light from a single exposure light source through a large number of unit lenses thereof to virtually transform the single exposure light source into a large number of point light sources so as to eliminate the irregularity in illuminance on the mask through integration of lights transmitted through the individual unit lenses. Consequently, if the filter is mounted on the fly eye lens, the number of unit lenses serving to eliminate the irregularity in illuminance is reduced, and the irregularity necessarily increases. In addition, the small aperture of the filter increases proximity effect.
Further, the oblique illumination method is ineffective for an isolated pattern such as an end pattern of repetition patterns or a contact hole, while it is effective for repetition patterns of line and space forming an image through interference. Therefore, in anticipation of dimensional changes, it is necessary to modify the pattern to be larger or smaller at the time of designing the mask. However, in producing the device employing the design rule of not more than 0.35 .mu.m, the modification is extremely small in scale and actually imposes a great amount of burden onto the mask designing and production.